This invention relates to a fabrication process of a thin-film device and also to a thin-film device, and specifically to a process for fabricating a thin-film device, which is useful in a liquid crystal display, organic electroluminescence display or the like, by forming the thin-film device on an original substrate having high heat resistance and then transferring it onto an application substrate and also to the thin-film device obtained by the fabrication process.
In recent years, thin-film devices are receiving a demand for thickness reduction, weight reduction and solidness under the influence of a move toward smaller equipment in which they are used. A limitation is, however, imposed on substrates for use in fabrication because thin-film devices are fabricated in a high-temperature vacuum environment. Employed, for example, in a liquid crystal display which makes use of thin-film transistors are silica substrates capable of withstanding temperatures up to 1,000° C. or glass substrates capable of withstanding temperatures up to 500° C. Thickness reductions of these substrates have been studied but, insofar as silica substrates or glass substrates are used, the substrates have to be reduced in size to cope with a reduction in rigidness so that the productivity is reduced. Further, a reduction in the thickness of a substrate immediately leads to a significant reduction in solidness, thereby developing a practical problem. As is appreciated from the foregoing, there is a difference between the performance required for an original substrate and the performance required upon actually using the thin-film device. Attempts have also been made to fabricate thin-film transistors directly on plastic substrates which permit thickness reduction, weight reduction and solidness improvement. These attempts, however, involve significant difficulties from the standpoint of the maximum withstand temperatures of the plastic substrates.
Investigations have, therefore, been made on techniques for transferring a thin-film device, which has been formed on an original substrate having a high maximum withstand temperature, onto an application substrate. For this transfer, it is necessary to separate only the thin-film device from the original substrate. Certain methods have been proposed for this purpose, including provision of a removable layer and subsequent etching of the removable layer with a chemical solution to separate a thin-film device layer and an original substrate from each other (see, for example, PCT International Application No. WO02/084739, page 9 and FIG. 2) and removal of an original substrate in its entirety by etching (see, for example, ibid., page 9 and FIG. 1D).
For the removal of an original substrate with a chemical solution subsequent to the formation of a thin-film device on the original substrate, it is necessary to form a protective layer with a thin film before the formation of the device layer such that the chemical solution is prevented from penetrating to the device layer. However, a thin-film layer formed by sputtering, vapor deposition or CVD is generally accompanied by a problem that it contains more or less pinholes and a chemical solution may penetrate to a device layer through the pinholes to damage the device layer with the chemical solution.